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Fix #4041: Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros #5142

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Fix #4041: Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros #5142

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46 changes: 26 additions & 20 deletions include/nlohmann/detail/macro_scope.hpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,12 @@
#define JSON_HAS_CPP_11
#endif

// Since C++20 the preprocessor supports __VA_OPT__(x) which expands to x only
// when __VA_ARGS__ is non-empty. We use this in NLOHMANN_DEFINE_TYPE_* macros
// to avoid generating invalid code when no member arguments are passed.
// On pre-C++20 compilers __VA_OPT__ is unavailable; zero-member invocations
// were already ill-formed on those compilers, so there is no regression.

#ifdef __has_include
#if __has_include(<version>)
#include <version>
Expand Down Expand Up @@ -416,9 +422,9 @@
*/
#define NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -428,9 +434,9 @@
*/
#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Type, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { __VA_OPT__(const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -440,7 +446,7 @@
*/
#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(Type, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -450,9 +456,9 @@
*/
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Type, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -462,9 +468,9 @@
*/
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(Type, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { __VA_OPT__(const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -474,7 +480,7 @@
*/
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(Type, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -484,9 +490,9 @@
*/
#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(Type, BaseType, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -496,9 +502,9 @@
*/
#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT(Type, BaseType, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType&>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType&>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
friend void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); __VA_OPT__(const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -508,7 +514,7 @@
*/
#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE(Type, BaseType, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }
friend void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -518,9 +524,9 @@
*/
#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(Type, BaseType, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -530,9 +536,9 @@
*/
#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT(Type, BaseType, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) } \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
void from_json(const BasicJsonType& nlohmann_json_j, Type& nlohmann_json_t) { nlohmann::from_json(nlohmann_json_j, static_cast<BaseType&>(nlohmann_json_t)); __VA_OPT__(const Type nlohmann_json_default_obj{}; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__))) }

/*!
@brief macro
Expand All @@ -542,7 +548,7 @@
*/
#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(Type, BaseType, ...) \
template<typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0> \
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) }
void to_json(BasicJsonType& nlohmann_json_j, const Type& nlohmann_json_t) { nlohmann::to_json(nlohmann_json_j, static_cast<const BaseType &>(nlohmann_json_t)); __VA_OPT__(NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__))) }

// inspired from https://stackoverflow.com/a/26745591
// allows calling any std function as if (e.g., with begin):
Expand Down
59 changes: 59 additions & 0 deletions test_empty_macro.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,59 @@
// Standalone compile test for issue #4041
// Tests that zero-member NLOHMANN_DEFINE_TYPE_* macros compile and work correctly.
#include <cassert>
#include <string>
#include <nlohmann/json.hpp>

// --- Intrusive (friend functions inside the class) ---
struct EmptyIntrusive
{
bool operator==(const EmptyIntrusive&) const { return true; }
NLOHMANN_DEFINE_TYPE_INTRUSIVE(EmptyIntrusive)
};

struct EmptyIntrusiveWithDefault
{
bool operator==(const EmptyIntrusiveWithDefault&) const { return true; }
NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(EmptyIntrusiveWithDefault)
};

struct EmptyIntrusiveOnlySerialize
{
NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(EmptyIntrusiveOnlySerialize)
};

// --- Non-intrusive (free functions in the same namespace) ---
struct EmptyNonIntrusive
{
bool operator==(const EmptyNonIntrusive&) const { return true; }
};
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(EmptyNonIntrusive)

struct EmptyNonIntrusiveWithDefault
{
bool operator==(const EmptyNonIntrusiveWithDefault&) const { return true; }
};
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(EmptyNonIntrusiveWithDefault)

struct EmptyNonIntrusiveOnlySerialize {};
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(EmptyNonIntrusiveOnlySerialize)

int main()
{
// to_json produces {}
assert(nlohmann::json(EmptyIntrusive{}).dump() == "{}");
assert(nlohmann::json(EmptyIntrusiveWithDefault{}).dump() == "{}");
assert(nlohmann::json(EmptyIntrusiveOnlySerialize{}).dump() == "{}");
assert(nlohmann::json(EmptyNonIntrusive{}).dump() == "{}");
assert(nlohmann::json(EmptyNonIntrusiveWithDefault{}).dump() == "{}");
assert(nlohmann::json(EmptyNonIntrusiveOnlySerialize{}).dump() == "{}");

// from_json round-trips successfully
nlohmann::json j = nlohmann::json::object();
auto e1 = j.get<EmptyIntrusive>(); (void)e1;
auto e2 = j.get<EmptyIntrusiveWithDefault>(); (void)e2;
auto e3 = j.get<EmptyNonIntrusive>(); (void)e3;
auto e4 = j.get<EmptyNonIntrusiveWithDefault>(); (void)e4;

return 0;
}
93 changes: 93 additions & 0 deletions tests/src/unit-udt_macro.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -460,6 +460,43 @@ class derived_person_only_serialize_private : person_without_default_constructor
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE(derived_person_only_serialize_private, person_without_default_constructor_1, hair_color)
};

// Zero-member types for issue #4041 regression: macros must compile and
// produce valid (empty) JSON objects when no member arguments are given.
struct empty_type_intrusive
{
bool operator==(const empty_type_intrusive& /*rhs*/) const { return true; }
NLOHMANN_DEFINE_TYPE_INTRUSIVE(empty_type_intrusive)
};

struct empty_type_intrusive_with_default
{
bool operator==(const empty_type_intrusive_with_default& /*rhs*/) const { return true; }
NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(empty_type_intrusive_with_default)
};

struct empty_type_intrusive_only_serialize
{
NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(empty_type_intrusive_only_serialize)
};

struct empty_type_non_intrusive
{
bool operator==(const empty_type_non_intrusive& /*rhs*/) const { return true; }
};
// NOLINTNEXTLINE(misc-use-internal-linkage)
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(empty_type_non_intrusive)

struct empty_type_non_intrusive_with_default
{
bool operator==(const empty_type_non_intrusive_with_default& /*rhs*/) const { return true; }
};
// NOLINTNEXTLINE(misc-use-internal-linkage)
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(empty_type_non_intrusive_with_default)

struct empty_type_non_intrusive_only_serialize {};
// NOLINTNEXTLINE(misc-use-internal-linkage)
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(empty_type_non_intrusive_only_serialize)

} // namespace persons

TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE", Pair, // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
Expand Down Expand Up @@ -707,3 +744,59 @@ TEST_CASE_TEMPLATE("Serialization of non-default-constructible classes via NLOHM
R"([{"age":1,"hair_color":"brown","name":"Erik"},{"age":2,"hair_color":"black","name":"Kyle"}])"));
}
}

// Regression test for issue #4041: zero-member NLOHMANN_DEFINE_TYPE_* macros
// must compile and produce valid (empty) JSON objects.
TEST_CASE_TEMPLATE("Zero-member NLOHMANN_DEFINE_TYPE_* macros produce empty JSON objects (issue #4041)", Json, // NOLINT
nlohmann::json, nlohmann::ordered_json)
{
SECTION("NLOHMANN_DEFINE_TYPE_INTRUSIVE with zero members")
{
persons::empty_type_intrusive obj{};
Json j = obj;
CHECK(j.dump() == "{}");
persons::empty_type_intrusive obj2 = j.template get<persons::empty_type_intrusive>();
CHECK(obj2 == obj);
}

SECTION("NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT with zero members")
{
persons::empty_type_intrusive_with_default obj{};
Json j = obj;
CHECK(j.dump() == "{}");
persons::empty_type_intrusive_with_default obj2 = j.template get<persons::empty_type_intrusive_with_default>();
CHECK(obj2 == obj);
}

SECTION("NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE with zero members")
{
persons::empty_type_intrusive_only_serialize obj{};
Json j = obj;
CHECK(j.dump() == "{}");
}

SECTION("NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE with zero members")
{
persons::empty_type_non_intrusive obj{};
Json j = obj;
CHECK(j.dump() == "{}");
persons::empty_type_non_intrusive obj2 = j.template get<persons::empty_type_non_intrusive>();
CHECK(obj2 == obj);
}

SECTION("NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT with zero members")
{
persons::empty_type_non_intrusive_with_default obj{};
Json j = obj;
CHECK(j.dump() == "{}");
persons::empty_type_non_intrusive_with_default obj2 = j.template get<persons::empty_type_non_intrusive_with_default>();
CHECK(obj2 == obj);
}

SECTION("NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE with zero members")
{
persons::empty_type_non_intrusive_only_serialize obj{};
Json j = obj;
CHECK(j.dump() == "{}");
}
}